EP0708357B1

EP0708357B1 - Optical parametric circuit

Info

Publication number: EP0708357B1
Application number: EP95402273A
Authority: EP
Inventors: Kunihiko Mori; Toshio Morioka; Masatoshi Saruwatari
Original assignee: Nippon Telegraph and Telephone Corp
Current assignee: Nippon Telegraph and Telephone Corp
Priority date: 1994-10-12
Filing date: 1995-10-11
Publication date: 2000-05-24
Anticipated expiration: 2015-10-11
Also published as: EP0708357A3; US5604618A; EP0708357A2; DE69517122T2; DE69517122D1

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical parametric circuit used for wavelength conversion of optical signals, parametric amplification of optical signals, optical phase conjugation (spectral inversion) and for all-optical switching by utilizing the third-order optical parametric effect induced in a nonlinear optical medium, and to optical circuits using the same, for example, optical logic circuits, optical time-division multi/demultiplexers and/or optical sampling circuits.

2. Description of the Related Art

ELECTRONICS LETTERS, Vol. 29, no 19, 17 September 1993, pages 1708-1710, INOUE K et al "Multichannel Frequency Conversion experiment using Fibre Four-Wave mixing" describes a conventional optic parametric circuit.
Figure 53 shows a configuration of a conventional optical parametric circuit.
This figure illustrates a process of generating the third-order optical parametric effect by coupling a signal wave S (of carrier angular frequency ω_s) shown in Figure 54A with pump waves P₁, P₂ (of carrier angular frequencies ω_P1, ω_P2) shown in Figure 54B into an optical wavelength-division multiplexer 11 (refer to Figure 54C) followed by a simultaneous inputting into a nonlinear optical medium 12 and propagating therefrom to induce the third-order optical parametric effect. The signal wave S is thus amplified (which is denoted by S'), and, through a four-wave mixing process, four-wave- mixing (shortened to FWM hereinbelow) wave F having a carrier angular frequency ω_f is generated (refer to Figure 54D).
Here, the carrier angular frequencies ω_S, ω_P1, ω_P2, ω_f of the signal wave S, pump waves P₁, P₂ and FWM wave F, are governed by the law of conservation of energy as expressed in the following equation: ωS + ωf = ωP1 + ωP2
The FWM wave F has a mirror symmetry with the spectrum of signal wave S with respect of the carrier angular frequency (ω_{P 1} + ω_P2)/2, and functions also as the optical phase conjugation wave for the signal wave S. In other words, this optical parametric circuit can function as a phase conjugation wave generation circuit. Figure 55 shows a circuit configuration using a degenerate pump wave P (carrier angular frequency ω_p), and Figures 56A to 56D show the spectra of the optical waves corresponding to the spectra shown in Figures 54A to 54D. In the drawings, the signal wave S and FWM wave F are shown as right-angle triangular mirror images, for illustrative purposes, to indicate the fact that the FWM wave F is the phase conjugate wave of the signal wave S.
Such an optical parametric circuit can also serve as a wavelength conversion circuit to perform simultaneous wavelength conversion of each wavelength of wavelength-division multiplexed signals. For example, upon injection of an N number of signal waves S₁∼S_N (carrier angular frequency ω_S1∼ω_SN), FWM waves F₁∼F_N are generated (carrier angular frequency ω_f1∼ω_fN where ω_fj=ω_P1+ω_P2-ω_Sj for j=1∼N) thus providing simultaneous wavelength conversion of each wave of the wavelength-division multiplexed signals. Figure 57 shows a circuit configuration based on a degenerate pump wave P (carrier angular frequency ω_P). Figures 58A to 58D show the spectra corresponding to the spectra shown in Figures 56A to 56D. In these figures, filled and unfilled triangles are used for showing the correspondence between signal waves S₁∼S_N and FWM waves F₁∼F_N.
An example of application of the parametric circuit as an optical amplification circuit is shown in Figure 59. Propagation patterns of each wave to the output port of the nonlinear optical medium 12 are the same as those presented in Figures 54A∼54D. The amplified signal S' is shown in Figure 54D.
Using the configuration shown in Figures 53, 55 and 57, signal wave S (S₁∼S_N), pump waves (P₁, P₂ and P) and FWM waves (F₁∼F_N) are all output colinearly from the nonlinear optical medium 12, therefore, to obtain only the FWM waves F (F₁∼F_N), it is necessary to employ a wavelength filter 13 which passes only those waves having carrier angular frequencies ω_f(ω_f1∼ω_fN). The output spectra from the wavelength filter 13 are shown in Figures 54E, 56E and 58E. By using the configuration as an optical amplification circuit of Figure 59, it is necessary to use a wavelength filter 29 which passes only the amplified signal wave S' of carrier angular frequency ω_S. The output spectrum from the wavelength filter 29 is shown in Figure 60.
When the optical parametric circuit is to be used as an FWM wave generator, it is necessary to pack the carrier angular frequencies ω_S, ω_P1 and ω_P2 to increase the conversion gain (expressed as FWM wave intensity/signal wave intensity) of signal wave S to FWM wave F, as well as increase the pump wave intensity. Similarly, when the optical parametric circuit is to be used as an optical parametric amplifier, it is necessary to pack the carrier angular frequencies, ω_S, ω_P1, ω_P2, and increase the pump wave intensity to increase the amplification gain of the signal wave.
Therefore, it is necessary for the wavelength filter 13 to possess a capability to suppress the pump wave intensity, which is higher relative to the FWM wave intensity, but pass the FWM waves having carrier angular frequencies which are closely packed with respect to the pump waves. However, it is difficult to sufficiently suppress the pump wave using only one wavelength filter, and therefore, it is general practice to employ a multi-stage optical filter. Therefore, the FWM wave or the signal wave suffers loss of the power and limitation of the bandwidth, and the circuit arrangement has been complicated. In the case of using the optical parametric circuit as the optical amplification circuit, there have been similar problems.
Also, in the conventional optical parametric circuits, when a phase conjugate signal (i.e. FWM wave F) is generated upon injection of an signal wave S, the carrier angular frequency is shifted from ω_S to ω_f (=ω_P1+ω_P2-ω_S), using the examples of Figures 56A and 56E. This presents a problem that when using the optical parametric circuit as a phase conjugation circuit in an optical fiber transmission system, the carrier angular frequency of transmitted optical signals is changed by passing through the phase conjugation circuit.
Furthermore, when the optical parametric circuit is to be used as a simultaneous wavelength conversion circuit for wavelength-division multiplexed signals, if the carrier angular frequencies ω_Sm, ω_Sn of the two signal waves S_m, S_n and the carrier angular frequencies ω_P1, ω_P2 of the pump waves are related in such a way to satisfy the equation: ωSm + ωSn = ωP1 + ωP2 then the carrier angular frequencies of the FWM waves F_m and F_n are set to ω_Sn and ω_Sm, meaning that the effect of interchanging the carrier angular frequencies of the signal waves would be obtained. However, in the conventional optical parametric circuits, the carrier angular frequencies of the FWM wave F_m becomes equal to that of the signal wave S_n, and similarly, the carrier angular frequencies of the FWM wave F_n becomes equal to that of the signal wave S_m. Therefore, it is not possible to separate the FWM waves F_m and F_n from the signal waves S_n and S_m using any wavelength filtering devices.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an optical parametric circuit to enable separation of four-wave-mixing wave(s) from signal wave(s) and pump waves without using wavelength filtering means, or to enable a separation of a four-wave-mixing wave from either signal wave(s) or pump waves, and to apply this optical circuit to functional devices.
The first object to separate generated four-wave-mixing wave(s) from signal wave(s) and pump waves is achieved in an optical parametric circuit comprising: a nonlinear optical medium for generating waves having carrier angular frequencies ω_fj(=ω_P1+ω_P2-ω_Sj; j=1, 2, ..N) from waves having carrier angular frequencies ω_Sj, ω_P1, and ω_P2; an optical directional coupler having a first input port, second input port, first output port and second output port for separating waves having carrier angular frequencies ω_Sj, ω_P1, ω_P2 and ω_fj into respective waves of an equal intensity and a polarization state; a first dispersive optical medium having a length dimension L₁ and a propagation constant K₁(ω) for connecting the first output port of the optical directional coupler with one end of the nonlinear optical medium; a second dispersive optical medium having a length dimension L₂ and a propagation constant K₂(ω) for connecting the second output port of the optical directional coupler with the other end of the nonlinear optical medium; wherein signal waves having carrier angular frequencies ω_S1, ω_S2, ...ω_SN and pump waves having carrier angular frequencies ω_P1, ω_P2 are injected into the first input port of the optical directional coupler so as to output four-wave-mixing waves ω_fj generated in the nonlinear optical medium from the second input port of the optical directional coupler, and wherein the carrier angular frequencies ω_Sj, ω_P1, ω_P2, and ω_fj are related by an expression: {K2(ωfj)+K2(ωSj)- K2(ωP1)-K2(ωP2)}L2 ={K1(ωfj)+K1(ωSj)-K1(ωP1)-K1(ωP2)}L1+(2n-1±α)π where n is an integer and an allowable error α is in a range 0≤α<1/2.
The optical parametric circuit configuration presented above enables the separation of the generated four-wave-mixing waves (FWM waves) from the signal waves and the pump waves, without using wavelengh-division optical demultiplexing means such as wavelength filters, by using the dispersive optical media which satisfy the specified conditions.
Accordingly, bandwidth limitations imposed by wavelength filters do not apply, and ultrafast processing of optical signals becomes possible and close packing of the carrier angular frequencies of the signal and pump waves enables to improve the generation efficiency of FWM waves.
The optical parametric circuit of the present invention also enables a heretofore unachieved task of separating optical phase conjugation waves, having carrier angular frequencies equal to or near the carrier angular frequencies of the signal waves, from the signal and pump waves.
The parametric circuit of the present invention also enables a heretofore unachieved task of wavelength interchange of wavelength-division multiplexed signal waves.
Even if optical noise components are included in the signal waves and pump waves, they are eliminated together with the signal and pump waves, thus improving the signal to noise ratio of FWM waves (phase conjugate waves and wavelength converted waves) by eliminating superposition of the noise signal components thereon.
Also, the second object to separate FWM wave(s) from either pump waves or signal wave(s) is achieved by using the same configuration as the above optical parametric circuit wherein the pump waves are injected into the first input port of the optical directional coupler, while the signal waves are injected into the second input port.
If the carrier angular frequencies ω_Sj, ω_P1, ω_P2, and ω_fj are related by an expression: {K2(ωfj)+K2(ωSj)-K2(ωP1)-K2(ωP2)}L2 ={K1(ωfj) +K1(ωSj)-K1(ωP1)-K1(ωP2)}L1+(2n±α)π the FWM waves generated in the nonlinear optical medium are output with the signal waves amplified in the nonlinear optical medium from the second input port, and separated from the pump waves which are output from the first input port.
On the other hand, if the carrier angular frequencies ω_Sj, ω_P1, ω_P2, and ω_fj are related by an expression: {K2(ωfj)+K2(ωSj)-K2(ωP1)-K2(ωP2)}L2 ={K1(ωfj)+K1(ωSj)-K1(ωP1)-K1(ωP2)}L1+(2n-1±α)π the FWM waves generated in the nonlinear optical medium are output with the pump waves from the first input port, and separated from the signal waves amplified in the nonlinear optical medium which are output from the second input port.
In the above cases, by using appropriate optical dispersive media, it is possible to couple the signal wave(s) and the pump waves and to separate the generated FWM wave(s) from either the pump waves or the signal wave(s) using the optical demultiplexing means such as wavelength filters. Even if optical noise components are contained in the pump waves input, the optical noise components are not output with either the generated FWM waves or the amplified signal waves.
It should be noted that signal waves of carrier angular frequencies ω_Sj (j=1, 2, ..N) include a single frequency signal wave having a singular frequency ω_S1 or ω_S, and, of course, in this case, the output FWM wave having a singular frequency ω_f (=ω_P1+ω_P2-ω_S) is output. Furthermore, either of the lengths L₁ and L₂ of the first and second dispersive optical media used in the parametric circuit of the present invention may be zero, i.e. the present invention includes those cases in which only one dispersive optical medium is used.
An FWM circuit using the so-called Sagnac interference device but without using the dispersive optical media is known in "A fiber frequency shifter with broad bandwidth, high conversion efficiency, pump and pump ASE cancellation, and rapid tunability for WDM optical networks", IEEE Photonics Technology Letters, Eric A. Swanson and John D. Moores, vol. 6, pp260-263, (1994). In this FWM circuit, the pump waves and the signal wave are injected into the different input ports of an optical directional coupler, and the generated FWM wave is output with the signal wave. Therefore, when the signal waves and the FWM waves have the same or similar carrier angular frequencies, it is not possible to isolate the FWM waves. This FWM circuit is therefore inapplicable to applications such as spectral inversion circuit with no wavelength shift or wavelength interchanging circuit of WDM signal waves.
As explained above, by using the optical parametric circuit of the present invention, it is possible to perform high efficiency, high S/N ratio, and wide-bandwidth optical processing such as wavelength conversion, phase conjugation generation (spectral inversion), optical signal amplification and all-optical switching, without the influence of high-power pump waves. All-optical switching includes such applications as optical pulse multiplexing circuit, optical pulse demultiplexing circuit, and optical waveform sampling circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic drawing of the configuration of a Embodiment 1 of the optical parametric circuit of the present invention.
Figures 2A ∼ 2C present the spectra of each wave in Embodiment 1.
Figures 3A ∼ 3C present the spectra of each wave in Variation 1 of Embodiment 1.
Figure 4 is a configuration of the optical parametric circuit in Variation 2 of Embodiment 1.
Figures 5A and 5B present the spectra of each wave in Variation 2 of Embodiment 1.
Figure 6 is a configuration of the optical parametric circuit in Variation 3 of Embodiment 1.
Figures 7A ∼ 7C present the spectra of each wave in Variation 3 of Embodiment 1.
Figures 8A and 8B present spectra of each wave in a Variation 4 of Embodiment 1 to facilitate separation of four-wave-mixing wave.
Figure 9 is a configuration of the optical parametric circuit in Variation 5 of Embodiment 1.
Figure 10A and 10B present the spectra of each wave in Variation 5 of Embodiment 1.
Figure 11 is a schematic drawing of the configuration of Embodiment 2 of the optical parametric circuit of the present invention.
Figure 12 is a configuration of the optical parametric circuit in Variation 1 of Embodiment 2.
Figure 13 is a schematic drawing of the configuration of Embodiment 3 of the optical parametric circuit of the present invention.
Figures 14A and 14B present the spectra of each wave in Embodiment 3.
Figure 15 is a schematic drawing of the configuration of Embodiment 4 of the optical parametric circuit of the present invention.
Figure 16 is a schematic drawing of the configuration of Embodiment 5 of the optical parametric circuit of the present invention.
Figures 17A and 17B illustrate the time-resolved spectra of each wave in Embodiment 6 of the present invention.
Figures 18A and 18B illustrate the time-resolved spectra of each wave in Embodiment 6 of the present invention.
Figure 19 illustrates the time-resolved spectra of each wave in Embodiment 7 of the present invention.
Figure 20 illustrates the time-resolved spectra of each wave in Embodiment 7 of the present invention.
Figure 21 illustrates the time-resolved spectra of each wave, which was obtained by a practical operation example of the optical phase conjugation circuit of the present invention.
Figure 22 is a schematic drawing of the configuration of Embodiment 8 of the optical parametric circuit of the present invention.
Figures 23A ^∼ 23D show optical waveforms corresponding to each carrier frequency in Embodiment 8.
Figure 24 presents a schematic drawing of the configuration of Variation 1 of Embodiment 8 of the optical parametric circuit of the present invention.
Figures 25A ∼ 25D show optical waveforms corresponding to each carrier frequency in Variation 1 of Embodiment 8.
Figure 26 is a schematic drawing of the configuration of Embodiment 9 of the optical parametric circuit of the present invention.
Figures 27A ∼ 27E show optical waveforms corresponding to each carrier frequency in Embodiment 9.
Figure 28 is a schematic drawing of the configuration of Embodiment 10 of the optical parametric circuit of the present invention.
Figures 29A ∼ 29D show optical waveforms corresponding to carrier frequency in Embodiment 10.
Figures 30A and 30B are schematic drawings of the configurations of Embodiment 11 of the optical parametric circuit of the present invention.
Figures 31A ∼ 31G present the spectra of each wave in Embodiment 11.
Figure 32 presents a schematic drawing of the configuration of Variation 1 of Embodiment 11 of the optical parametric circuit of the present invention.
Figures 33A ∼ 33C present the spectra of each wave in Variation 1 of Embodiment 11.
Figure 34 is a schematic drawing of the configuration of Embodiment 12 of the optical parametric circuit of the present invention.
Figure 35 is a schematic drawing of the configuration of Embodiment 13 of the optical parametric circuit of the present invention.
Figure 36 is a schematic drawing of the configuration of Embodiment 14A of the optical parametric circuit of the present invention.
Figure 37 shows the spectrum of an amplified output signal.
Figure 38 is a schematic drawing of the configuration of Embodiment 14B of the optical parametric circuit of the present invention.
Figure 39 shows the spectra of a pump wave and an FWM wave both of which are output from port 20a.
Figures 40A and 40B are schematic drawings of the configuration of Embodiment 15 of the optical parametric circuit of the present invention.
Figure 41 shows the spectrum of an FWM wave output through a wavelength filter 13.
Figure 42 is a schematic drawing of the configuration of an optical parametric circuit in Embodiment 16 of the present invention.
Figures 43A ∼ 43E show the optical waveforms corresponding to each carrier frequency in Embodiment 16.
Figure 44 is a schematic drawing of the optical parametric circuit in Variation 1 of Embodiment 16 of the present invention.
Figures 45A ∼ 45F show the optical waveforms corresponding to each carrier frequency in Variation 1 of Embodiment 16.
Figure 46 is a schematic drawing of the optical parametric circuit in Variation 2 of Embodiment 16 of the present invention.
Figures 47A and 47B show the optical waveforms corresponding to each carrier frequency in Variation 2 of Embodiment 16.
Figure 48 is a schematic drawing of the configuration of an optical parametric circuit in Embodiment 17 of the present invention.
Figures 49A ∼ 49F show the optical waveforms corresponding to each carrier frequency in Embodiment 17.
Figure 50 is a schematic drawing of the configuration of an optical parametric (waveform sampling) circuit in Embodiment 18 of the present invention.
Figure 51 is similarly a schematic drawing of the configuration of an optical parametric (waveform sampling) circuit in Embodiment 18 of the present invention.
Figures 52A ∼ 52D show each wave in the optical parametric (waveform sampling) circuit in Embodiment 18.
Figure 53 is a schematic diagram of a conventional optical parametric circuit.
Figures 54A ∼ 54E show the spectra of each wave in the circuit shown in Figure 53.
Figure 55 is a schematic diagram of a conventional optical phase conjugation circuit.
Figures 56A ∼ 56E show the spectra of each wave in the circuit shown in Figure 55.
Figure 57 is a schematic diagram of a conventional simultaneous wavelength conversion circuit for wavelength-division multiplexed signal waves.
Figures 58A ∼ 58E show the spectra of each wave in the circuit shown in Figure 57.
Figure 59 is a schematic diagram of a conventional optical parametric amplifier circuit.
Figure 60 shows the spectrum of the signal wave in the circuit shown in Figure 59.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments will be explained with reference to the drawings.

Embodiment 1

Figure 1 is a schematic diagram of the optical parametric circuit of the present invention being used as an FWM wave generation circuit.
With reference to Figure 1, a nonlinear optical medium 12 is inserted between two dispersive optical media 21, 22 to which ports 20c, 20d of a 2x2 optical directional coupler 20 are respectively connected. The optical directional coupler 20 has a capability to separate optical waves having carrier frequencies (shortened to carrier frequency hereinbelow) ω_S, ω_P1, ω_P2, ω_f(= ω_P1+ω_P2-ω_S) at equal intensities and equal polarization state. The length of the dispersive optical medium 21 which joins port 20c with the nonlinear optical medium 12 is denoted by L₁ and the propagation constant therein by K₁(ω), and the length of the dispersive optical medium 22 which joins port 20d and the nonlinear optical medium 12 is denoted by L₂ and the propagation constant therein by K₂(ω). A typical example of the dispersive optical medium would be an optical waveguide.
A signal wave S having the carrier frequency ω_S and the pump waves P₁, P₂ having the carrier frequencies ω_P1, ω_P2 (refer to Figure 2A) are injected into the port 20a of the optical directional coupler 20, and are separated at ports 20c, 20d into waves of equal intensities and polarization states, respectively. However, the phases of the signal wave S_L and the pump waves P_1L, P_2L output from port 20d, which is the cross-port with respect to port 20a, are retarded by 90 degrees from the phases of the signal wave S_R and the pump waves P_1R, P_2R output from port 20c, which is the through-port with respect to port 20a. Therefore, if the complex electrical field amplitude (hereinafter shortened to complex amplitude) of the signal wave S_R at port 20c is denoted by A_S, and the complex amplitudes of the pump waves P_1R, P_2R at port 20c are denoted by A_P1, A_P2, then the complex amplitude of the signal wave S_L at port 20d is given by: A_S·exp(-iπ/2), and the complex amplitudes of the pump waves P_1L, P_2L are given by A_P1·exp(-iπ/2), A_P2·exp(-iπ/2), respectively.
Reviewing the above, the signal wave S_R and the pump waves P_1R, P_2R separated to port 20c of the optical directional coupler 20 propagates in the direction of the dispersive optical medium 21→ nonlinear optical medium 12→ dispersive optical medium 22→ port 20d, i.e. in the clockwise direction. Here, the complex amplitudes A_SR, A_P1R, and A_P2R of the clockwise signal S_R and the pump waves P_1R, P_2R which are input from port 20c via dispersive optical medium 21 into nonlinear optical medium 12 are expressed as follows. ASR = AS·exp(-iK1(ωS)L1) APIR = AP1·exp(-iK1(ωP1)L1) AP2R = AP2·exp(-iK1(ωP2)L1)
In the meanwhile, the signal wave S_L and the pump waves P_1L, P_2L separated to port 20d of the optical directional coupler 20 propagate in the direction of the dispersive optical medium 22→ nonlinear optical medium 12→ dispersive optical medium 21→ port 20c, i.e. in the counter-clockwise direction. Here, the complex amplitudes A_SL, A_P1L and A_P2L of the counter-clockwise signal S_L and the pump waves P_1L, P_2L which are input from port 20d via dispersive optical medium 22 into nonlinear optical medium 12 are expressed as follows. ASL = AS·exp(-iK2(ωS)L2-iπ/2) AP1L =AP1·exp(-iK2(ωP1)L2-iπ/2) AP2L =AP2·exp(-iK2(ωP2)L2-iπ/2)
Within the nonlinear optical medium 12, due to the third-order optical parametric effect induced by the propagation of the clockwise signal wave S_R and the pump waves P_1R and P_2R, a clockwise FWM wave F_R having the carrier frequency ω_f (=ω_P1+ω_P2-ω_S) is generated (refer to Figure 2B). Similarly, due to the third-order parametric effect induced by the propagation of the counter-clockwise signal wave S_L and the pump waves P_1L and P_2L, a counter-clockwise FWM wave F_L having the carrier frequency ω_f is generated. The clockwise and counter-clockwise components again enter the optical directional coupler 20 and produce interference.
In this case, although the signal waves S_R, S_L and the pump waves P_1R, P_2R, P_1L and P_2Lundergo phase shift due to chromatic dispersion while propagating in the nonlinear optical medium 12 and the dispersive optical media 21, 22 and due to nonlinear effects (self-phase modulation, cross-phase modulation) in the nonlinear optical medium 12, there is no phase shift difference between the counter-clockwise and clockwise waves themselves. Therefore, this optical circuit functions essentially as a so-called optical loop mirror (refer to D.B. Mortimore, "Fiber loop reflectors", IEEE Journal of Lightwave Technology, vol. 6, pp.1217-1224, 1988) to the signal wave S and the pump waves P₁, P₂, and in principle, 100 % of the signal wave S and the pump waves P₁, P₂ is output to port 20a.
Here, the complex amplitude A_fR of a clockwise FWM wave F_R generated by the clockwise signal wave S_R and the clockwise pump waves P_1R, P_2R (expressed by the expressions (1), (2) and (3)) is given by the following expression. AfR = C · ASR* · AP1R · AP2R = C · AS * · AP1 · AP2 · exp[i(K1(ωS)-K1(ωP1)-K1(ωP2))L1]
Also, the complex amplitude A_fL of a counter-clockwise FWM wave F_L generated by the counter-clockwise signal wave S_L and the counter-clockwise pump waves P_1L, P_2L (expressed by the expressions (4), (5) and (6)) is given by the following expression. AfL = C · ASL* · AP1L · AP2L = C · AS* · AP1 · AP2 · exp[i(K2(ωS)-K2(ωP1)-K2(ωP2))L2- iπ/2] where A_S* is the complex conjugate of A_S and C is the square root of the conversion efficiency given by an expression: (intensity of FWM wave/(intensity of signal wave S x intensity of pump wave P₁ x intensity of pump wave P₂)).
The complex amplitudes A_d, A_c of the clockwise and counter-clockwise FWM waves F_R, F_L, as each wave arrives at the ports 20d, 20c through the respective dispersive optical medium 22, 21, are given by the following expressions. Ad = AfR · exp(-iK2(ωf)L2) = C · AS* · AP1 · AP2 · exp[-i(K1(ωf)L1+K2(ωf)L2)] · exp(iΔK1L1) AC = AfL · exp(-iK1(ωf)L1) = C · AS* · AP1 · AP2 · exp[-i(K1(ωf)L1+K2(ωf)L2)] · exp(iΔK2L2-iπ/2) where ΔK1 =K1(ωS)+K1(ωf)-K1(ωP1)-K1(ωP2) and ΔK2 = K2(ωS)+K2(ωf)-K2(ωP1)-K2(ωP2)
In the optical directional coupler 20, the clockwise and counter-clockwise FWM waves F_R, F_L interfere, and the complex amplitudes A_a, A_b of the FWM waves output from ports 20a, 20b are given by the following expressions. Aa ∝ Ac + Ad · exp(-iπ/2) ∝ exp(iΔK2L2) + exp(iΔK1L1) Ab ∝ Ac · exp(-iπ/2) + Ad ∝ -exp(iΔK2L2) + exp(iΔK1L1)
It follows that when the dispersive optical media 21, 22 fulfil the condition that: ΔK2L2 = ΔK1L1 + (2n-1)π where n is an integer, then, in principle, 100 % of FWM wave F can be output to port 20b, as illustrated in Figure 2C.
The above results mean that it becomes possible to separate the FWM wave F generated in the nonlinear optical medium 12 from the signal wave S and the pump waves P₁, P₂ without the use of such wavelength-division demultiplexing means as wavelength filters.
Even when some optical noise is contained in the signal wave and the pump waves P₁, P₂, the optical noise is output together with the signal wave S and the pump waves P₁, P₂ from port 20a. There is no danger of the noise components becoming mixed in the FWM wave F.
It may be noted that taking into account certain allowable error α (0≤α<1/2)in expression (15), the following expression is obtained: ΔK2L2 = ΔK1L1 + (2n-1±α)π
In this case, depending on the magnitude of the allowable error α, the output intensity level of FWM wave F output to port 20b is lowered; however, the signal wave S and the pump waves P₁, P₂ are never output to port 20b. Therefore, no problems can occur in practice.
When using a degenerate pump wave, the substitutions are made in the above expressions such that ω_P1=ω_P2, and A_P1=A_P2. Also, when only one dispersive optical medium is being used, either L₁ or L₂ in the above expressions is set equal to zero.

Variation 1 of Embodiment 1

The first variation of the Embodiment 1 of the FWM wave generation circuit has the same circuit configuration as that shown in Figure 1, in which ω_S and ω_f become symmetrical with respect to (ω_P1+ω_P2)/2, and the generated FWM wave F becomes a phase conjugation wave of an input signal wave S. As shown in Figure 3A, the carrier frequencies of the signal wave S and the pump waves P₁, P₂ are related such that ω_P1<ω_S<ω_P2 and ω_S = (ω_P1+ω_P2)/2, then as shown in Figure 3B, the carrier frequencies of the clockwise wave S_R and the FWM wave (phase conjugation wave) F_R become equal. The same result is produced with respect to the counter-clockwise signal wave S_L. In this case (Variation 1 of Embodiment 1) also, the separation of the signal wave S and the FWM wave (phase conjugation wave) F is possible according to the principle presented above (refer to Figure 3C).

Variation 2 of Embodiment 1

Similarly, an example of using the degenerate wave in the parametric circuit of Figure 1 will be explained, i.e., the carrier frequencies ω_P1 and ω_P2 of the pump waves are such that ω_P1=ω_P2=ω_P.
As shown in Figure 4, when a signal wave S of carrier frequency ω_S and the pump wave of carrier frequency ω_P are injected into port 20a of the optical directional coupler 20, an FWM wave F of carrier frequency ω_f (=2ω_P-ω_S) is generated in the nonlinear optical medium 12 (refer to Figure 5B). The FWM wave F is output from port 20b of the optical directional coupler 20, while the signal wave S and the pump wave P are output from port 20a of the optical directional coupler 20.

Variation 3 of Embodiment 1

Similarly in the optical parametric circuit of Figure 1, a case of injecting signal waves S₁, S₂, ..S_N (carrier frequencies ω_S1, ω_S2, ..ω_SN) multiplexed in the wavelength region and the pump waves P₁, P₂ of carrier frequencies ω_P1, ω_P2 into port 20a of the optical directional coupler 20 will be explained. However, the intensity of each signal wave is chosen so as to be able to ignore the mutual nonlinear interaction of the signal waves.
As shown in Figure 6, the wavelength-division multiplexed signal waves (WDM signal waves) S₁, S₂, ..S_N and the pump waves P₁, P₂ (refer to Figure 7A) are separated to port 20c of the optical directional coupler 20, and propagate successively through the dispersive optical medium 21→ nonlinear optical medium 12→ dispersive optical medium 22→ port 20d in the clockwise direction. Here, the complex amplitude A_SjR of the clockwise jth signal wave S_jR of the clockwise WDM signal waves S_1R, ..S_NR input from port 20c into the nonlinear optical medium 12 through the dispersive optical medium 21 is given by the following expression. ASjR = ASj · exp (-ik1(ωSj)L1)
In the meanwhile, the jth signal wave S_j of the WDM signal waves S₁, ..S_N and the pump waves P₁, P₂ separated to port 20d of the optical directional coupler 20 propagate through the dispersive optical medium 22→ nonlinear optical medium 12→ dispersive optical medium 21→ port 20c, in the counter-clockwise direction. Here, the complex amplitude A_SjL of the counter-clockwise jth signal wave S_jL input from port 20d into the nonlinear optical medium 12 through the dispersive optical medium 22 is expressed as follows. ASjL = ASj · exp(-iK2(ωSj)L2-iπ/2)
The third-order parametric effect induced by the propagation of the clockwise jth signal wave S_jR and the pump waves P_1R, P_2R in the nonlinear optical medium 12, generates the jth FWM wave F_jR of carrier frequency ω_fj (=ω_Pi+ω_P2-ω_Sj) (refer to Figure 7B). Similarly, the third-order parametric effect induced by the propagation of the counter-clockwise jth signal wave S_jL and the pump waves P_1L, P_2L generates the jth FWM wave F_jL of the carrier frequency ω_fj. These clockwise and counter-clockwise propagating components again enter the optical directional coupler 20 to produce interference and, in principle, 100 % of the jth signal wave S_j and the pump waves P₁, P₂ is output to port 20a.
Here, the complex amplitude A_fjR of the clockwise jth FWM wave F_jR generated by the clockwise WDM signal wave S_jR and the pump waves P_1R, P_2R is expressed as follows. AfjR = C · ASj* · AP1 · AP2 · exp[i(K1(ωSj)-K1(ωP1)-K1(ωP2))L1]
Similarly, the complex amplitude A_fjL, of the counter-clockwise jth FWM wave F_jL generated by the counter-clockwise jth signal wave S_jL and the pump waves P_1L, P_2L is expressed as follows. AfjL = C · ASj* · AP1 · AP2 · exp[i(K2(ωSj)-K2(ωP1)-K2(ωP2))L2 - iπ/2]
The complex amplitudes A_dj, A_cj of the clockwise and counter-clockwise jth FWM waves F_jR, F_jL upon arrival at ports 20d, 20c through the dispersive optical media 22, 21 are given by the following expressions. Adj = C · ASj* · AP1 · AP2 · exp[-i(K1(ωfj)L1+K2(ωfj)L2)] · exp(iΔK1jL1) Acj = C · ASj* · AP1 · AP2 · exp[-i(K1(ωfj)L1+K2(ωfj)L2)] · exp(iΔK2jL2-iπ/2) where ΔK1j = K1(ωSj)+K1(ωfj)-K1(ωP1)-K1(ωP2) and ΔK2j =K2(ωSj)+K2(ωfj)-K2(ωP1)-K2(ωP2)
In the optical directional coupler 20, the clockwise and counter-clockwise jth waves F_jR, F_jL interfere, and the complex amplitudes A_aj, A_bj of the jth FWM wave F_j output to ports 20a, 20b are given by the following expressions. Aaj ∝ exp(iΔK2jL2) + exp (iΔK1jL1) Abj ∝ -exp(iΔK2jL2) + exp(iΔK1jL1)
It follows that when the dispersive optical media 21, 22 fulfil the condition that: ΔK2jL2 = ΔK1jL1 + (2n-1)π where n is an integer, then, in principle, 100 % of the jth FWM wave F_j can be output to port 20b (refer to Figure 7C), thus completely separating the jth signal wave S_j, and the pump waves P₁, P₂ therefrom.
By configuring the optical parametric circuit as demonstrated above which uses the dispersive optical media 21, 22 satisfying the expression (15') for each j=1,2, ..N, it becomes possible to completely separate the FWM waves F₁, ..FN obtained by simultaneous wavelength conversion of each wavelength of the WDM signal waves S₁, ..S_n, from the WDM signal waves S₁, ..S_N and the pump waves P₁, P₂ without the use of such wavelength separation means such as wavelength filters.
Also, if the carrier frequencies ω_Sm, ω_Sn of the two signal waves S_m, S_n and the carrier frequencies ω_P1, ω_P2 of the pump waves P₁, P₂ are related in such a way to satisfy the equation: ωSm + ωSn = ωP1 + ωP2 then the carrier frequencies of the FWM waves F_m, F_n are set to ω_Sn and ω_Sm, and wavelength interchange of the signal waves will be obtained.
It may be noted that taking into account certain allowable error α (0≤α<1/2)in expression (15'), the following expression is obtained: ΔK2jL2 = ΔK1jL1 + (2n-1±α)π
In this case, depending on the magnitude of the allowable error α, the output intensity level of the jth FWM wave F_j output to port 20b is lowered; however, the WDM signal waves S₁, ..S_N, and the pump waves P₁, P₂ are never output to port 20b.
When using degenerate pump waves, the substitutions are made in the above expressions such that ω_P1= ω_P2, and A_P1=A_P2. Also, when only one dispersive optical medium is being used, either L₁ or L₂ in the above expressions is set equal to zero.

Variation 4 of Embodiment 1

The fourth variation of the Embodiment 1 of the FWM wave generation circuit has the same circuit configuration as that shown in Figure 6, in which the carrier frequencies ω_S1, ω_S2, ..ω_SN, ω_P1, ω_P2 of the WDM signal waves S₁, S₂, ..S_N and the pump waves P₁, P₂ satisfy the following expression: ωP1 < ωSj < ωP2 for j=1,2,..N.
It should be noted that, in addition to the FWM wave F_j of a carrier frequency ω_P1+ω_P2-ω_Sj generated in the nonlinear optical medium 12, unwanted FWM waves of carrier frequencies 2ω_P1-ω_Sj, 2ω_P2-ω_Sj are produced by the degenerate pump waves P₁, P₂, at a spacing |ω_P1-ω_P2|. If the carrier frequencies of the WDM signal wave S₁, S₂, ..S_N and the pump waves P₁, P₂ satisfy the expression ω_P1< ω_Sj< ω_P2 (J=1,2, ...N), as shown in Figure 8A, the FWM wave F_j of carrier frequency ω_P1+ω_P2-ω_Sj excited by the non-degenerate pump waves is generated between the frequencies ω_P1, ω_P2, as shown in Figure 8B. The unwanted FWM waves excited by the degenerate pump waves of carrier frequencies 2ω_P1-ω_Sj, 2ω_P2-ω_Sj are generated in a range outside the frequencies ω_P1, ω_P2. Thus, the wanted FWM waves can be readily separated from the unwanted FWM waves.

Variation 5 of Embodiment 1

A fifth variation is based similarly on the parametric circuit of Figure 1, in which a simultaneous wavelength conversion of WDM signal waves is performed by utilizing the degenerate pump wave. Referring to Figure 9, the pump wave P is the degenerate pump wave of carrier frequency ω_P=ω_P1=ω_P2 which is injected into port 20a of the optical directional coupler 20.
As shown in Figure 9, when the WDM signal waves S₁, S₂, ..S_N of carrier frequencies ω_Sj (j=1, 2, ..N) and the pump wave P of carrier frequency ω_P (refer to Figure 10A) are injected into port 20a, FWM waves F_1, F₂ ..F_N of carrier frequencies ω_fj (=2ω_P-ω_Sj) are produced in the nonlinear optical medium 12. The FWM waves F_1, F₂, ..F_N are output from port 20b of the directional optical coupler 20 (refer to Figure 10B), and the WDM signal waves S₁, S₂, ..S_N and the pump wave P are output from port 20a.
Here, if the carrier frequencies ω_Sm, ω_Sn of the two signal waves S_m, S_n and the carrier frequencies ω_P of the pump wave P are related in such a way to satisfy the equation: ωSm + ωSn = 2ωP then the carrier frequencies of the FWM waves F_m, F_n are set to ω_Sn and ω_Sm, and wavelength interchange of the signal waves S_m, S_n will be performed.

Embodiment 2

Figure 11 shows a circuit configuration of Embodiment 2 of the optical parametric circuit of the present invention.
The feature of Embodiment 2 is that the circuit components of Figure 1 are made as a polarization-maintaining type, that is, the optical directional coupler 23, nonlinear optical medium 24 and the dispersive optical media 25, 26 are all polarization-maintaining devices. This configuration is also applicable to the phase conjugation generation circuit presented in Variation 1 of the first embodiment, and the simultaneous wavelength conversion circuit of the WDM signal waves presented in Variation 2 of the first embodiment.
It should be noted that in the circuit of non-polarization-maintaining type, changes in the polarization states of the signal wave(s) and the pump waves injected will cause the leakage of the signal wave(s) and the pump waves into the FWM wave(s) output from the circuit. Therefore, by making each component to be polarization-maintaining, it is possible to suppress polarization crosstalk within the circuit and improve the isolation of the signal wave(s) and the pump waves with respect to the FWM wave(s). Such polarization-maintaining devices can be a birefringent material such as polarization-maintaining optical fiber, and inputting signal wave(s) and pump waves polarized along the optic axis of the material will enable to preserve the polarization states of the optical waves in the optical circuit. In addition to such polarization-maintaining devices, it is effective in suppressing the polarization crosstalk to insert single-polarization elements such as polarizers, in input/output section or loop section of the circuit.

Variation 1 of Embodiment 2

Figure 12 is an optical parametric circuit having a polarization dispersion compensator 27, such as a birefringent waveguide, added to the circuit shown in Figure 11. It is also possible to apply the same circuit configuration to the phase conjugation generation circuit presented in Variation 1 of Embodiment 1 and the simultaneous wavelength conversion circuit for WDM signal waves presented in Variation 3 of Embodiment 1.
Changes in the polarization state of the signal wave causes distortions in the waveform of the output FWM wave(s) due to the birefringence (polarization dispersion) in the circuit components in the circuit of polarization-maintaining type. Also, when the conversion gains (expressed as a ratio of intensity of FWM wave to that of signal wave) in the two optic axes (polarization axes) of the polarization-maintaining nonlinear optical medium are not the same, changes in polarization states in the input signal wave(s) will cause variations in the intensity of the output FWM wave(s). Therefore, a polarization dispersion compensator 27, such as a birefringent waveguide as shown in Figure 12 is used to compensate the total polarization dispersion in the circuit as shown in Figure 11. By so doing, it is possible to prevent waveform degradation of the FWM wave caused by the changes in the polarization state of the signal wave.
The polarization compensator 27 is, as shown in this figure, disposed at the FWM wave output section (port 23b of the optical directional coupler 23), but it may also be disposed in the input path of the signal wave and at the injection section (port 23a of the coupler 23) of the signal wave or within the loop (between ports 23c and 23d of the coupler 23). Also, a method as disclosed in a US Patent No. 5,357,359 ("All-optical polarization independent optical time division multiplexer and demultiplexer with birefringence compensation") may also be used, i.e., the two components of each electrical field of the signal wave(s), pump waves and FWM wave(s) polarized along two optic axes of the circuit shown in Figure 11 are interchanged from one optic axis to another optic axis at points in the optical parametric circuit which compensate the total polarization dispersion within the circuit.
To equalize the conversion gain of the signal wave with respect to the two polarization axes of the polarization-maintaining nonlinear optical medium, when a polarization-maintaining nonlinear optical medium such as polarization-maintaining optical fiber having the same conversion gains with respect to the two polarization axes is used, the pump waves are to be injected into each polarization axis at a 1:1 intensity ratio. When using a device which exhibits two different conversion gains with respect to the two polarization axes, such as semiconductor optical amplifiers, it is necessary to adjust the intensity ratio of the pump waves so as to obtain the same conversion gain in each of the polarization axes.
By so doing, it is possible to make the operation of the optical parametric circuit polarization-insensitive.

Embodiment 3

Figure 13 shows a circuit configuration of Embodiment 3 of the optical parametric circuit of the present invention used as an FWM wave generation circuit provided with an optical multiplexer for coupling the signal wave with the pump wave.
The feature of this embodiment is that a wavelength-division multiplexer 11 has been provided, in front of port 20a of the optical directional coupler 20, to the circuit configuration shown in Figure 1, for coupling the signal wave S (shown in Figure 14A) with the pump waves P₁, P₂ (shown in Figure 14B). The carrier frequency ω_f of the output FWM wave F can be tuned by selecting the carrier frequencies ω_P1, ω_P2 of the pump waves P₁, P₂. The circuit configuration of Embodiment 3 is applicable to both the phase conjugation generation circuit presented in Variation 1 of Embodiment 1 as well as to the simultaneous wavelength conversion circuit for WDM signal waves presented in Variation 3 of Embodiment 1.

Embodiment 4

Figure 15 shows a circuit configuration of Embodiment 4 of the optical parametric circuit of the present invention used as an FWM wave generation circuit for blocking the return signal wave and the pump waves.
The feature of this embodiment is that a blocking device has been provided to the circuit of Figure 1 at port 20a of the optical directional coupler 20 for blocking the signal waves S and the pump waves P₁, P₂ which are output from port 20a of the optical directional coupler 20. The circuit configuration of Embodiment 4 is also applicable to both the phase conjugation generation circuit presented in Variation 1 of Embodiment 1 as well as to the simultaneous wavelength conversion circuit for WDM signal waves presented in Variation 3 of Embodiment 1.
The signal wave S and the pump waves P₁, P₂ return to input port 20a, thus very high intensity pump waves P₁, P₂ could exert undesirable influence on the generators for signal wave and pump waves. Therefore, the blocking device 28 is disposed in front of port 20a of the optical directional coupler 20 to block the returning signal wave S and the pump waves P₁, P₂ which are output from port 20a. The blocking device 28 could be an optical isolator or an optical circulator.

Embodiment 5

Figure 16 shows a circuit configuration of a fifth embodiment of the optical parametric circuit of the present invention provided with a device for suppressing unwanted wave components leaking into the output FWM wave.
The feature of this embodiment is that a filtering device has been provided to the circuit of Figure 1 after port 20b of the optical directional coupler 20 for suppressing the unwanted signal waves leaking into the FWM wave F output from port 20b of the optical directional coupler 20. The circuit configuration of Embodiment 5 is also applicable to both the phase conjugation generation circuit presented in Variation 1 of Embodiment 1 as well as to the simultaneous wavelength conversion circuit for WDM signal waves presented in Variation 3 of Embodiment 1.
The reasons for causing leaking of signal waves(s) and pump waves into output FWM wave(s) are, for example, asymmetry in the splitting ratio (other than 50 : 50) of the optical directional coupler 20 and polarization crosstalk between signal wave S and pump waves P₁, P₂ which may occur in the optical directional coupler 20, nonlinear optical medium 12 and dispersive optical media 21, 22. Other possibilities include the unwanted FWM waves of carrier frequencies 2ω_P1-ω_S, 2ω_P2ω_S generated simultaneously when generating FWM waves of carrier frequencies ω_f(=ω_P1+ω_P2-ω_S) from the signal waves and pump waves of carrier frequencies ω_S, ω_P1, ω_P2, respectively.
To suppress the output of unwanted waves from port 20b, the circuit shown in Figure 16 provides a wavelength filter 13 which passes only the wavelengths having carrier frequencies ω_f at port 20b of the optical directional coupler 20 for generating FWM wave F.

Embodiment 6

Embodiment 6 of the optical parametric circuit is an application to an optical phase conjugation circuit using angular-modulated pump waves or to a simultaneous wavelength conversion of WDM signal waves using angular-modulated pump waves.
The feature of this embodiment is to perform phase modulation or frequency modulation of two pump waves P₁, P₂ in the circuits shown in Variation 1 of Embodiment 1 and Variation 4 of Embodiment 1.
When cw pump waves with narrow spectral widths are used, nonlinear back-scattering waves, which are caused by stimulated Brillouin scattering or induced diffraction grating generated in the nonlinear optical medium 12 and the dispersive optical media 21, 22, are output from port 20b of the directional optical coupler 20. According to this embodiment, by performing angular modulation to widen the spectral bandwidth of the pump waves, the threshold intensity of the pump waves for causing back-scattering is increased, thereby permitting the use of higher intensity pump waves.
Especially, as shown in Figures 17A and 18A, when angular modulations which are mutually conjugate are applied to the two pump waves, the instantaneous carrier frequency (ω_P1+ω_P2)/2 is held constant, thereby preventing variations in the carrier frequencies of the generated FWM wave F (or F_j). This is demonstrated in Figures 17B and 18B. In Figures 18A and 18B, filled and unfilled circles are used for showing the correspondence between the WDM signal waves and the FWM waves.
In general, if the amount of angular modulation of the signal wave is denoted by _S(t) (or _Sj(t)) and the angular modulations of the pump waves by _P1(t) and _P2(t), and when they are related as in the following expression: P1(t) + P2(t) - S(t) = 0 (or P1(t) + P2(t) - Sj(t) = 0) it is possible to prevent variations in the carrier frequencies of the FWM waves generated.

Embodiment 7

Embodiment 7 of the optical parametric circuit is an application to a phase conjugation generation circuit or a simultaneous wavelength conversion of WDM signal waves using intensity-modulated pump waves.
The feature of the circuit of this embodiment is the addition of intensity modulation to provide intensity modulations synchronized to the signal wave S (or WDM signal waves S₁, ..S_N) to the two pump waves P₁, P₂ in the circuits of Variation 1 of Embodiment 1 and Variation 4 of Embodiment 1.
According to the circuit of this embodiment, it is also possible to prevent the generation of back-scattering waves as in the circuit presented in Embodiment 6. Figure 19 corresponds to Figure 17A, and Figure 20 corresponds to Figure 18A, respectively, for Embodiments 6 and 7. In this embodiment, it is possible to raise the peak intensity of the pump waves while keeping the average intensity of the pump waves constant.
It is also permissible to use combined modulation of the intensity modulation presented in this embodiment and the angular modulation presented in Embodiment 6.
The actual example of the operation of the phase conjugation generation circuit of the present invention will be explained with reference to Figure 21. In the circuit of this example, polarization-maintaining optical fibers are used to serve as the nonlinear optical medium 24 and the dispersive optical medium 25 (both of which components are polarization-maintaining types). Also, phase modulation to the two pump waves P₁, P₂ is performed. Time-resolved spectra for the signal wave S and the pump waves P₁, P₂ injected into port 23a of the optical directional coupler 23 are shown in observed image (a). In observed image (b) are shown time-resolved spectra of the phase conjugation wave (FWM wave) F output from port 23b of the optical directional coupler 23. It is demonstrated that it is possible to generate phase conjugation wave F having the same wavelength as that of the signal wave S.
Together with the image of the phase conjugation wave F presented in the observed image (b), an image H₁ is seen at the same wavelength as the pump wave P₂. The image H₁ is generated from a wavelength conversion of pump wave P₁ caused by signal wave S acting as the degenerate pump wave, and appears at port 23b of the directional optical coupler 23. Similarly, an image H₂ is seen at the same wavelength as the pump wave P₁. The image H₂ is generated from a wavelength conversion of pump wave P₂ caused by signal wave S acting as the degenerate pump wave, and appears at port 23b of the directional optical coupler 23. The images H₁ and H₂ both have different wavelengths from that of the phase conjugation wave F, and therefore, they can readily be eliminated using wavelength filters.

Embodiment 8

Figure 22 shows a circuit configuration of Embodiment 8 which is an application of the parametric circuit to a dual-input optical gate circuit (optical AND-circuit).
The feature of the circuit of this embodiment is that the dual-input single-output optical gate circuit (optical AND-circuit) is realized by coupling of an optical input signal S_in with an optical gate signal G with the use of a wavelength-division multiplexer 11, which is input, through an optical isolator 28 into port 20a of the optical directional coupler 20.
In this embodiment, the optical input signal S_in (refer to Figure 23A) is a pulsed signal wave S of carrier frequency ω_S, the optical gate signal G (refer to Figure 23B) is a pulsed pump wave P of carrier frequency ω_P and the optical output signal S_out is the FWM wave F of carrier frequency ω_f(=2ω_P-ω_S). When the optical input signal S_in and the optical gate signal G are input into the nonlinear optical medium 12 so as to overlap each other, an optical output signal S_out is generated in a timeslot t₂ (refer to Figure 23C), and the optical output signal S_out is output from port 20b (refer to Figure 23D). However, it is noted that a wavelength conversion (ω_S→ω_f) takes place in the optical gate circuit of this embodiment.

Variation 1 of Embodiment 8

Figure 24 shows a first variation of Embodiment 8, which is an application of the parametric circuit to a triple-input optical gate circuit (optical AND-circuit).
The feature of the circuit of this variation is to present a triple-input one-output optical gate circuit (optical AND-gate) which does not result in wavelength conversion while using the same optical gate circuit shown in Figure 22.
In this embodiment, the optical input signal S_in (refer to Figure 25A) of carrier frequency ω_S is related to the carrier frequencies ω_P1, ω_P2 of optical gate signals G₁, G₂ which are modulated as shown in Figure 25B by the following expression. 2ωS = ωP1+ωP2
When the optical input signal S_in and the optical gate signals G₁, G₂ are input into the nonlinear optical medium 12 so as to overlap each other, the carrier frequency of the generated FWM wave F becomes ω_s, that is, the carrier frequencies of the optical input signal S_in and the optical output signal S_out become the same (refer to Figure 25C). The optical input signal S_in input into port 20a of the optical directional coupler 20 together with optical gate signals G₁, G₂ is output again from port 20a. On the other hand, the optical output signal S_out of carrier frequency ω_S is output from port 20b of the optical directional coupler 20 (refer to Figure 25D).

Embodiment 9

Figure 26 shows the circuit configuration of Embodiment 9 which is an application of the parametric circuit to an optical time-division demultiplexer.
The feature of this embodiment is that, by connecting optical gate circuits shown in Figure 22 in a multi-stage configuration, an optical time-division demultiplexer is realized for separating a time-division multiplexed (TDM) optical signal.
In this embodiment, the first-stage optical gate circuit demultiplexes two channels; wherein a TDM optical input signal S_in which is a pulsed signal wave of carrier frequency ω_S and which contains four channels in the timeslots (t₁, t₂, t₃, t₄)(refer to Figure 27A) and an optical gate signal G₁ which is a pulsed pump wave of carrier frequency ω_P and which is active in the timeslots (t₂, t₄)(refer to Figure 27B) are input, and an optical output signal S_out1 which is a generated FWM wave of carrier frequency ω_f(=2ω_P-ω_S) in the respective timeslots (t₂, t₄) is output (refer to Figure 27C).
In the second-stage optical gate circuit, an optical input signal which is the optical output signal S_out1 of the first-stage optical gate circuit and an optical gate signal G₂ which is a pulsed pump wave of carrier frequency ω_P and which is active in the timeslots (t₃, t₄)(refer to Figure 27D) are input, and an optical pulse signal S_out2 which is a generated FWM wave of carrier frequency ω_S(=2ω_P-ω_f) in the timeslot (t₄) is output (refer to Figure 27E). The procedure is able to demultiplex the TDM optical input signal S_in to extract one channel in the timeslot in which both optical gate signals G₁, G₂ are active.
It should be noted that in the first-stage optical gate circuit, the TDM optical input signal S_in of carrier frequency ω_S generates an optical output signal (FWM wave) S_out1 of carrier frequency ω_f. Similarly, in the second-stage optical gate circuit, the optical input signal (the optical input signal S_out1) of carrier frequency ω_f generates an optical output signal (FWM wave) S_out2 of carrier frequency ω_S. Accordingly, this circuit of Embodiment 9 functions in the same way as the triple-input optical gate circuit of Variation 1 of Embodiment 8 in which the carrier frequencies remain in an optical input signal and an optical output signal as a whole, while the circuit of Embodiment 9 has the advantage of requiring only one carrier frequency ω_P for the optical gate signals G₁ and G₂.
It is also permissible to apply the multi-stage configuration to the triple-input optical gate circuit of Variation 1 of Embodiment 8 (refer to Figure 24).

Embodiment 10

Figure 28 shows the circuit configuration of Embodiment 10 which is an application of the parametric circuit to another type of optical time-division demultiplexer.
The feature of this embodiment is that, by combining the gate circuit of Figure 22 with a 1-input, 4-output wavelength-division demultiplexer 49, it is possible to present an optical time-division demultiplexer for simultaneous separation of four channels of time-division multiplexed optical pulse signals.
In the circuit of Figure 28, a TDM optical input signal S_in of carrier frequency ω_S which contains four channels in the timeslots (t₁, t₂, t₃, t₄)(refer to Figure 29A) is input together with optical gate signals G₁, G₂, G₃, G₄ which are respectively active in the timeslots (t₁, t₂, t₃, t₄) at different carrier frequencies ω_P1, ω_P2, ω_P3, ω_P4 (refer to Figure 29B), and optical output signals S_out1, S_out2, S_out3, S_out4 of different carrier frequencies ω_f1, ω_f2, ω_f3, ω_f4 are output (refer to Figure 29C) which are generated in degenerate four-wave mixing process between the TDM optical input signal S_in and each of the optical gate signals G₁, G₂, G₃, G₄, where ω_fk=2ω_Pk-ω_S (k=1,..4). The respective optical output signals S_out1, S_out2, S_out3, S_out4 are separated in the wavelength-division demultiplexer 29 into individual channels of carrier frequencies ω_f1, ω_f2, ω_f3, ω_f4, thereby achieving a simultaneous separation of four channels (refer to Figure 29D).
In this case, for k=1, 2, 3, 4, it is assumed that the relationship of the frequencies is given by the following expression. {K2(ωS)+K2(ωfk)-2K2(ωPk)}L2 = {K1(ωS)+K1(ωfk)-2K1(ωPk)}L1 + (2n-1±α)π
In the optical time-division demultiplexers shown above, it is also permissible to use non-degenerate pump waves as optical gate signals.

Embodiment 11

Figure 30A is a schematic diagram of the optical parametric circuit of Embodiment 11 of the present invention, showing the flow of optical waves.
In this embodiment, a signal wave S (refer to Figure 31A) of carrier frequency ω_S is injected into port 20b of the optical directional coupler 20, and is divided into ports 20c, 20d at equal intensities and polarization states, respectively. The phase of the signal wave S_R output from port 20c, which is a cross port of port 20b, is retarded by 90 degrees with respect to that of the signal wave S_L output from port 20d which is the through port of port 20b. Therefore, designating the complex amplitude of the output signal wave S_L at port 20d by A_S, the complex amplitude of the signal wave S_R at port 20c is given by A_S · exp(-iπ/2).
The pump waves P₁, P₂ (refer to Figure 31B) of carrier frequencies ω_P1, ω_P2 are injected into port 20a of the optical directional coupler 20, and are divided into ports 20c, 20d at equal intensities and polarizations. However, the phases of the pump waves P_1L, P_2L output from the cross port 20d are retarded by 90 degrees from the phases of the pump waves P_1R, P_2R output from the through port 20c, respectively. Therefore, designating the complex amplitudes of the pump waves P_1R, P_2R at port 20c by A_P1, A_P2 the complex amplitudes of the pump waves P_1L, P_2L at port 20d is given by A_P1 · exp(-iπ/2) and A_P2 · exp(-iπ/2), respectively.
Accordingly, the signal wave S_R and the pump waves P_1R, P_2R (refer to Figure 31C) output to port 20c of the directional coupler 20 propagate in the direction of the dispersive optical medium 21→ nonlinear optical medium 12→ dispersive optical medium 22→ port 20d, i.e. in the clockwise direction. Here, the complex amplitudes A_SR, A_P1R, and A_P2R of the clockwise signal S_R and the clockwise pump waves P_1R, P_2R which are input from port 20c via dispersive optical medium 21 into nonlinear optical medium 12 are expressed as follows. ASR = AS · exp(-iK1(ωS)L1-iπ/2) AP1R = AP1 · exp(-iK1(ωP1)L1) AP2R = AP2 · exp(-iK1(ωP2)L1)
In the meanwhile, the signal wave S_L and the pump waves P_1L, P_2L output to port 20d of the directional coupler 20 propagate in the direction of the dispersive optical medium 22→ nonlinear optical medium 12→ dispersive optical medium 21→ port 20c, i.e. in the counter-clockwise direction. Here, the complex amplitudes A_SL, A_P1L and A_P2L of the counter-clockwise signal S_L and the counter-clockwise pump waves P_1L, P_2L which are input from port 20d via dispersive optical medium 22 into nonlinear optical medium 12 are expressed as follows. ASL = AS · exp(-iK2(ωS)L2) AP1L = AP1 · exp(-iK2(ωP1)L2-iπ/2) AP2L = AP2 · exp(-iK2(ωP2)L2-iπ/2)
Within the nonlinear optical medium 12, due to the third- order optical parametric effect induced by the propagation of the clockwise signal wave S_R and the clockwise pump waves P_1L and P_2L, the signal wave S_R is amplified and a clockwise FWM wave F_R having the carrier frequency ω_f (=ω_P1+ω_P2-ω_S) is generated (refer to Figure 31D). Similarly, due to the third-order parametric effect induced by the propagation of the counter-clockwise signal wave S_L and the counter-clockwise pump waves P_1L and P_2L, the signal wave S_L is amplified and a counter-clockwise FWM wave F_L having the carrier frequency ω_f is generated. The clockwise and counter-clockwise wave components again enter the directional optical coupler 20 and produce interference.
The circuit of this embodiment also functions as an optical loop mirror with respect to the signal waves S and the pump waves P₁, P₂; therefore, in principle, 100 % of the amplified signal wave S' is output to port 20b and the pump waves P₁ and P₂ to port 20a, to enable the result of a complete separation of the amplified signal waves S', and the pump waves P₁ and P₂.
Here, the complex amplitude A_fR of a clockwise FWM wave F_R generated by the clockwise signal wave S_R and the clockwise pump waves P_1R, P_2R (expressed by the expressions (17), (18) and (19)) is given by the following expression. AfR = C · ASR* · AP1R · AP2R = C · AS* · AP1 · AP2 · exp[i(K1(ωS)-K1(ωP1)-K1(ωP2))L1+iπ/2]
Also, the complex amplitude A_fL of a counter-clockwise FWM wave F_L generated by the counter-clockwise signal wave S_L and the counter-clockwise pump waves P_1L, P_2L (expressed by the expressions (20), (21) and (22)) is given by the following expression. AfL = C · ASL* · AP1L · AP2L = C · AS* · AP1 · AP2 · exp[i(K2(ωS)-K2(ωP1)-K2(ωP2))L2- iπ] where C is the square root of the conversion efficiency previously explained above.
The complex amplitudes A_d, A_c of the clockwise and counter-clockwise FWM waves F_R, F_L, as each wave arrives at the ports 20d, 20c through the respective dispersive optical medium 22, 21, are given by the following expressions. Ad = AfR·exp(-iK2(ωf)L2) = C·AS*·AP1·AP2·exp[-i(K1(ωf)L1+K2(ωf)L2)] · exp (iΔK1L1+iπ/2) Ac = AfL·exp(-iK1(ωf)L1) = C·AS* · AP1·AP2·exp[-i(K1(ωf)L1+K2(ωf)L2)] · exp(iΔK2L2-iπ) where ΔK₁, ΔK₂ are the same as the expressions given in (11) and (12).
In the optical directional coupler 20, the clockwise and counter-clockwise FWM waves F_R, F_L interfere, and the complex amplitudes A_a, A_b of the FWM waves output to the ports 20a, 20b are given by the following expressions. Aa ∝ Ac + Ad·exp(-iπ/2) ∝ exp(iΔK2L2) - exp(iΔK1L1) Ab ∝ Ac · exp(-iπ/2) + Ad ∝ exp(iΔK2L2) + exp(iΔK1L1)
It follows that when the dispersive optical media 21, 22 fulfil the condition that: ΔK2L2 = ΔK1L1 + 2nπ where n is an integer, then, in principle, 100 % of FWM wave F can be output to port 20b with the amplified signal wave S' (refer to Figure 31E), thus completely isolating the pump waves P₁, P₂ (which are output to port 20a) therefrom.
Meanwhile, Figure 30B shows the state of propagation of the waves when the dispersive optical media 21, 22 satisfy the following condition. ΔK2L2 = ΔK1L1 + (2n-1)π where n is an integer. In this case, 100 % of the FWM wave F is output to port 20a together with pump waves P₁, P₂ (refer to Figure 31F) in principle, and the amplified signal wave S' is output to port 20b (refer to Figure 31G). This means that the amplified signal wave S' is completely separated from the FWM wave F and the pump waves P₁, P₂.
By configuring the optical parametric circuit as demonstrated above, when selecting the dispersive optical media 21, 22 to satisfy the expression (29), it becomes possible to separate the FWM wave F generated and the amplified signal wave S' from the pump waves P₁, P₂ without the use of such wavelength-division demultiplexing means as wavelength filters. On the other hand, when selecting the dispersive optical media 21, 22 to satisfy the expression (30), it becomes possible to separate the amplified signal wave S' from the FWM wave F and the pump waves P₁, P₂. This aspect of the circuit operation will be explained later in another embodiment.
It may be noted that taking into account certain allowable error α (0≤α<1/2)in expression (29), the following expression is obtained. ΔK2L2 = ΔK1L1 + (2n-1±α)π
In this case, depending on the magnitude of the allowable error α, the output intensity level of FWM wave F output to port 20b is lowered; however, the pump waves P₁, P₂ are never output to port 20b. Therefore, no problems can occur in practice.
Even when some optical noise is contained in the pump waves P₁, P₂, the optical noise is output together with the pump waves P₁, P₂ from port 20a, the optical noise becomes never mixed in the amplified signal wave S' and the FWM wave F.

Variation 1 of Embodiment 11

Figure 32 presents a first variation of Embodiment 11 in which a degenerate pump wave (carrier frequency ω_P=ω_P1=ω_P2) is used in the circuit of Embodiment 11.
In this embodiment, when a signal wave S shown in Figure 31A is injected from port 20b of the optical directional coupler 20, and the pump wave P (refer to Figure 33A) of carrier frequency ω_P is injected from port 20a, an FWM wave F_R of carrier frequency ω_f (=2ω_P-ω_S) is generated. Figure 33B shows the generated FWM wave F_R, the amplified signal wave S_R', and the pump wave P_R which are output from the nonlinear optical medium 12 in the clockwise direction. The same effect occurs in the counter-clockwise direction. The FWM wave Fand the amplified signal wave S' are output from port 20b, and the pump wave P from port 20a of the optical directional coupler 20.
The following embodiments are all explained in the case that the pump wave P is degenerate.
As in the first embodiment, in place of a signal wave S, WDM signal waves S₁, S₂, ..S_N of carrier frequencies ω_S1, ω_S2, .. ω_SN can be used to separate the WDM FWM wave F_j and the signal wave from the pump waves. As in the case presented in Embodiment 2 and shown in Figure 11, polarization-maintaining type configuration can be used to suppress polarization crosstalk within the circuit to improve the isolation of the pump waves with respect to the amplified signal wave S' and the FWM wave F. Further, by providing polarization compensating means as in the case of Variation 1 of Embodiment 2, polarization-insensitivity can be achieved. In the cases of the embodiments to follow, it is also possible to suppress polarization crosstalk by using polarization-maintaining type components.

Embodiment 12

Figure 34 shows a configuration of the optical parametric circuit of Embodiment 12.
The feature of this embodiment is to block the return pump wave P output from port 20a of the optical directional coupler 20 in the basic configuration presented in Embodiment 11.
In the parametric circuit of this embodiment, the pump wave P returns to the input port 20a. Therefore, very high intensity pump wave P could exert undesirable influence on the generator for the pump wave. An optical isolator 28 is disposed in front of port 20a of the optical directional coupler 20 for injection of the pump wave P to block the pump wave P output from port 20a. An optical isolator 28 may be replaced with an optical circulator.

Embodiment 13

Figure 35 shows a configuration of the optical parametric circuit of Embodiment 13.
The feature of this embodiment is to separate the signal wave S input into port 20b of the optical directional coupler 20 from the amplified signal wave S' and FWM wave F output from port 20b of the optical directional coupler 20 in the basic configuration presented in Embodiment 11.
In this optical parametric circuit, because an input signal wave S and an output signal wave S' and FWM wave F all pass through the same port 20b of the optical directional coupler 20, a means for separating them is necessary. This is achieved by disposing an optical circulator 38 on port 20b of the optical directional coupler 20 to separate the amplified signal wave S' and the FWM wave F from the input signal wave S.

Embodiment 14

Embodiment 14 is presented in two versions: 14A and 14B.

Embodiment 14A

Figure 36 shows a configuration of the optical parametric circuit of Embodiment 14A as an optical amplifier.
In this embodiment, in addition to the configuration of Embodiment 13, a wavelength filter 29 is used, which blocks FWM wave F and passes only the amplified signal wave S' (of carrier frequency ω_s), The pump wave P (of carrier frequency ω_p) may be either a cw (continuous wave) wave or an intensity-modulated wave synchronized with the signal wave S. When a signal wave S is injected into port 20b of the optical directional coupler 20 through the optical circulator 38, an amplified signal wave S' (refer to Figure 37) is output from port 20b through the optical circulator 38 and the wavelength filter 29.

Embodiment 14B

Figure 38 shows another configuration of the optical parametric circuit as an optical amplifier in Embodiment 14B.
The feature of this embodiment is the selection of propagation constants K₁(ω) and K₂(ω) and the lengths L₁ and L₂ of the respective dispersive optical media 21, 22 (expressed in Equation (30)) so that only an amplified signal wave S' is output from port 20b of the optical directional coupler 20, and separated from an FWM wave F and the pump wave P which are output from port 20a (refer to Figure 39). Therefore, the wavelength filter 29 used in Figure 36 to pass the amplified signal wave S' and suppress FWM wave F is unnecessary in principle.
In this embodiment, an optical circulator 38 is used to separate the amplified signal wave S' output from port 20b of the optical directional coupler 20 from the signal wave S input into port 20b of the optical directional coupler 20.

Embodiment 15

Figure 40A shows a circuit configuration of the optical parametric circuit applied to an FWM wave generation circuit in Embodiment 15.
In this embodiment, a wavelength filter 13 which passes only FWM waves F has been added to the circuit which was presented in Embodiment 13 and shown in Figure 35. When a signal wave S (carrier frequency ω_S) is input via the optical circulator 38 into port 20b of the optical directional coupler 20, an FWM wave F is output from port 20b through the optical circulator 38 and the wavelength filter 13 (refer to Figure 41). The carrier frequency ω_f(=2ω_P-ω_S) of the FWM wave F can be tuned by selecting the carrier frequency ω_P of the pump wave P.
As shown in Figure 40B, it is permissible to replace the optical circulator 38 and the wavelength filter 13 with a wavelength-division demultiplexer 30 which separates the FWM wave of carrier frequency ω_f from the signal waves S, S' of carrier frequency ω_S.
In Embodiments 14 and 15, even if the pump wave P leaks into port 20b, the wavelength filters 29, 13 are able to filter the leaked pump wave P, therefore, the isolation of pump wave P with respect to the FWM wave F is further enhanced. It is also permissible to dispose an additional wavelength filter which suppresses the pump wave P. If the circuit is to be a polarization-maintaining type, a polarizer is also effective in suppressing the leak of pump waves caused by polarization crosstalk to further enhance the performance.

Embodiment 16

Embodiment 16 is an application of the circuit presented in Embodiment 15 to a 2-input 1-output optical gate circuit (optical AND-circuit). Figure 42 shows the flow of optical signals in the circuit.
In this embodiment, an optical input signal S_in is input into port 20b of the optical directional coupler via the optical circulator 38 and an optical gate signal G is input into port 20a, where the optical input signal S_in is a pulsed signal wave of carrier frequency ω_S (refer to Figure 43A) and the optical gate signal G is a pulsed pump wave of carrier frequency ω_P (refer to Figure 43B). Then, an optical output signal S_out which is an FWM wave of carrier frequency ω_f(=2ω_P-ω_S) is produced and the optical input signal S_in' is amplified in the timeslots (t₂) in which the optical input signal S_in and the optical gate signal G overlap each other in the nonlinear optical medium 12. Figure 43C shows these optical signals in the clockwise direction. The optical output signal S_out and the amplified optical input signal S_in' are separated from the optical gate signal G and output from port 20b of the optical directional coupler 20 (refer to Figure 43D). The signals S_out and S_in' then pass through the optical circulator 38 and are further filtered with a wavelength filter 13 so as to block the amplified optical input signal S_in' and to output only the optical output signal S_out (refer to Figure 43E). Therefore, this circuit has the same function as the circuit explained in Embodiment 8 and shown in Figure 22.

Variation 1 of Embodiment 16

Figure 44 shows a circuit configuration of a Variation 1 of Embodiment 16. This is an application of the optical parametric circuit to an optical time-division demultiplexer for separating a TDM signal, by connecting optical gate circuits shown in Figure 42 in a multi-stage configuration.
In this variation, in the first-stage optical gate circuit, a TDM optical input signal S_in which is a pulsed signal wave of carrier frequency ω_S and which contains four channels in the timeslots (t₁, t₂, t₃, t₄)(refer to Figure 45A) and an optical gate signal G₁ which is a pulsed pump wave of carrier frequency ω_P and which is active in the timeslots (t₂, t₄)(refer to Figure 45B) are input, and an optical output signal S_out1 which is a generated FWM wave of carrier frequency ω_f(=2ω_P-ω_S) in the respective timeslots (t₂, t₄)(refer to Figure 45C) is output via a wavelength filter 13 which passes only the carrier frequency ω_f to block the amplified optical input signal S_in' (refer to Figure 45F).
In the second-stage optical gate circuit, a optical input signal which is the optical output signal S_out1 from the first-stage optical gate circuit and an optical gate signal G₂ which is a pulsed pump wave of carrier frequency ω_P and which is active in the timeslots (t₃, t₄)(refer to Figure 45D) are input, and an optical output signal S_out2 which is a generated FWM wave of carrier frequency ω_S(=2ω_P-ω_f) in the timeslot (t₄) is output via a wavelength filter 29 which passes only the carrier frequency ω_S to block the amplified optical input signal S_out1' (refer to Figure 45E).
The procedure enables to demultiplex the TDM optical input signal S_in to extract one channel in the timeslot in which both optical gate signals G₁ and G₂ are active, and presents the same function as the circuit of Embodiment 9.

Variation 2 of Embodiment 16

Figure 46 shows a circuit configuration of a Variation 2 of Embodiment 16. The feature of this configuration is that the wavelength filters 13, 29 in the optical time-division demultiplexer presented in Figure 44 are replaced with wavelength-division demultiplexers 30₁, 30₂.
In this variation, in the first-stage optical gate circuit, a TDM optical input signal S_in of carrier frequency ω_S which contains four channels in the timeslots (t₁, t₂, t₃, t₄)(same as is shown in Figure 45A) and an optical gate signal G₁ of carrier frequency ω_P which is active in the timeslots (t₂, t₄)(same as is shown in Figure 45B) are input, and an optical output signal S_out1 which is a generated FWM wave of carrier frequency ω_f(=2ω_P-ω_S) in the corresponding timeslots (t₂, t₄)(same as is shown in Figure 45C) is output together with the optical input signal S_in' amplified in the corresponding timeslots (t₂, t₄)(same as is shown in Figure 45F) from an optical circulator 38₁ to a wavelength-division demultiplexer 30₁ which separates the optical output signal S_out1 of carrier frequency ω_f from the amplified optical input signal S_in' of carrier frequency ω_s. The first-stage optical gate circuit, therefore, achieves the isolation of the two timeslots (t₂, t₄) of the TDM optical input signal S_in.
In the second-stage optical gate circuit, an optical input signal which is the amplified optical output signal S_in" output from the first-stage optical gate circuit and an optical gate signal G₂ of carrier frequency ω_P which is active in the timeslots (t₁, t₃)(refer to Figure 47A) are input, and an optical output signal S_out2 which is a generated FWM wave of carrier frequency ω_f(=2ω_P-ω_s) in the timeslots (t₁, t₃)(refer to Figure 47B) is output with the optical input signal S_in" amplified in the corresponding timeslots (t₁, t₃) from an optical circulator 38₂ to a wavelength-division demultiplexer 30₂ which separates the optical output signal S_out2 of carrier frequency ω_f from the amplified optical input signal S_in" of carrier frequency ω_s. The second-stage optical gate circuit, therefore, achieves the isolation of the two timeslots (t₁, t₃) of the TDM optical input signal S_in.
The procedure enables to demultiplex the TDM optical input signal S_in to extract channels in the timeslots in which respective optical gate signals G₁ and G₂ are active. The amplified optical input signal S_in" output from the second-stage circuit can also be used to demultiplex the TDM optical input signal S_in to extract any channels in additional stages of this optical gate circuit with the wavelength-division demultiplexer 30.

Embodiment 17

Figure 48 shows a circuit configuration of Embodiment 17, in which the parametric circuit of Figure 35 is connected in a multi-stage configuration to perform time-division multiplexing of input pulse signals S_in1, S_in2.
In this case, an optical clock signal C which is a pulsed signal wave of carrier frequency ω_S (refer to Figure 49A) is input through an optical circulator 38₁ into port 20b of the optical directional coupler 20₁ in the first-stage optical gate circuit. The first optical input signal S_in1 which is a pulsed pump wave of carrier frequency ω_P (refer to Figure 49B) is input into port 20a. At this time, an optical output signal S_out1 (FWM wave) of carrier frequency ω_f(=2ω_P-ω_S) is generated in the timeslots (t₂, t₄) of the first input signal S_in1 in the nonlinear optical medium 12₁ and is output together with the optical clock signal C' amplified in the active timeslots (t₂, t₄)(refer to Figure 49C) from the optical circulator 38₁.
The optical clock signal C' and the optical output signal S_out1 are input into port 20b of the optical directional coupler 20₂ through the optical circulator 38₂ of the second-stage optical gate circuit. In the meantime, the second optical input signal S_in2 which is a pulsed pump wave of carrier frequency ω_P (refer to Figure 49D) is input into port 20a. At this time, an optical output signal (FWM wave) S_out2 of carrier frequency ω_f(=2ω_P-ω_s) is generated in the active timeslots (t₁, t₃) of the second optical input signal S_in2 in the nonlinear optical medium 12₂ and is output from port 20b together with the optical clock signal C" amplified in the timeslots (t₁, t₃) (refer to Figure 49E).
Similarly, additional optical input signals can be multiplexed by further connecting additional optical gate circuits.
The wavelength filter 13 blocks the optical clock signal C", and passes only the TDM optical output signal S_out2 of carrier frequency ω_f (refer to Figure 49F). Accordingly, the optical circuit of this embodiment generates TDM optical output signal S_out2 of carrier frequency ω_f from the two optical input signals S_in2 of carrier frequency ω_S.

Embodiment 18

Figures 50, 51 show an application of the optical parametric circuit to an optical waveform sampling circuit for observing optical signals.
The feature of the operating mode of the parametric circuit of this embodiment is that a pulsed signal wave of carrier frequency ω_S is injected together with a pulsed pump wave of carrier frequency ω_P having a comparatively narrower pulse width (than that of the target pulse wave) to generate an FWM wave of carrier frequency ω_f(=2ω_P-ω_s). The parametric circuit is provided with a photodetector 40 to detect the FWM wave of carrier frequency ω_f and a display 41 as a display means for displaying the detected signal waveform from the photodetector.
In this embodiment, a pulsed pump wave P having a repetition frequency f+Δf is used to sample a pulsed signal wave S having a repetition frequency (bit rate) f, where the pulse width of the pump wave P is narrower than that of the pulsed signal wave S. The pulsed signal wave S of carrier frequency ω_S (refer to Figure 52A) and a pulsed pump wave P of carrier frequency ω_P (refer to Figure 52B) produce an FWM wave F of carrier frequency ω_f(=2ω_P-ω_s)(refer to Figure 52C) having a waveform in which the pulsed pump wave P is amplitude-modulated by the pulsed signal wave S. The envelop of the FWM wave F (shown by dashed curve in Figure 52C) having a repetition frequency Δf is obtained by detecting wave F with a photodetector 40 having a suitable bandwidth (refer to Figure 52D).
Figure 50 shows an optical waveform sampling circuit which uses the optical parametric circuit shown in Figure 22. Also, Figure 51 shows an optical waveform sampling circuit which uses the optical parametric circuit shown in Figure 40A. In Figures 50 and 51, the optical waveform sampling circuits are provided with a display 41 to display the waveform of the defected signal output from the photodetector 40.

Claims

An optical parametric circuit comprising:

a nonlinear optical medium (12) for generating optical waves having carrier angular frequencies ω_f1, ω_f2, ..ω_fN from optical waves having carrier angular frequencies ω_S1, ω_S2, ..ω_SN, ω_P1, and ω_P2, where ω_fj=ω_P1+ω_P2-ω_Sj(j=1,2, ..N), characterised by

an optical directional coupler (20) having a first input port (20a), second input port (20b), first output port (20c) and second output port (20d), for dividing optical waves having carrier angular frequencies ω_Sj, ω_P1, ω_P2 and ω_fj into two optical waves of equal intensities and polarization states, respectively;

a first dispersive optical medium (21) having a length dimension L₁ and a propagation constant K₁(ω), for connecting the first output port (20c) of said optical directional coupler with one end of said nonlinear optical medium;

a second dispersive optical medium (22) having a length dimension L₂ and a propagation constant K₂(ω), for connecting the second output port (20d) of said optical directional coupler with other end of said nonlinear optical medium;

wherein signal waves having carrier angular frequencies ω_S1, ω_S2, ..ω_SN and pump waves having carrier angular frequencies ω_P1, ω_P2 are injected into the first input port (20a) of said optical directional coupler so as to output four-wave-mixing waves having carrier angular frequencies ω_f1, ω_f2, ..ω_fN generated in said nonlinear optical medium from the second input port (20b) of said optical directional coupler, and wherein said carrier angular frequencies ω_Sj, ω_P1, ω_P2, and ω_fj are related by an expression: {K2(ωfj)+K2(ωSj)-K2(ωP1)-K2(ωP2)}L2 = {K1(ωfj)+K1(ωSj)-K1(ωP1)-K1(ωP2)}L1+(2n-1±α)π where n is an integer and an allowable error α is in a range 0≤α<1/2.
An optical parametric circuit comprising:

a nonlinear optical medium (12) for generating optical waves having carrier angular frequencies ω_f1, ω_f2, ..ω_fN from optical waves having carrier angular frequencies ω_S1, ω_S2, ..ω_SN, ω_P1, and ω_P2, where ω_fj=ω_P1+ω_P2-ω_Sj(j=1,2, ..N), characterised by

an optical directional coupler (20) having a first input port (20a), second input port (20b), first output port (20c) and second output port (20d), for dividing optical waves having carrier angular frequencies ω_Sj, ω_P1, ω_P2 and ω_fj into two optical waves of equal intensities and polarization states, respectively;

a first dispersive optical medium (21) having a length dimension L₁ and a propagation constant K₁(ω), for connecting the first output port (20c) of said optical directional coupler with one end of said nonlinear optical medium;

a second dispersive optical medium (22) having a length dimension L₂ and a propagation constant K₂(ω), for connecting the second output port (20d) of said optical directional coupler with other end of said nonlinear optical medium;

wherein pump waves having carrier angular frequencies ω_P1, ω_P2 are injected into the first input port (20a) of said optical directional coupler and signal waves having carrier angular frequencies ω_S1, ω_S2, ..ω_SN are injected into the second input port (20b) of said optical directional coupler so as to output said signal waves having carrier angular frequencies ω_S1, ω_S2, ..ω_SN amplified in said nonlinear optical medium together with four-wave-mixing waves having carrier angular frequencies ω_f1, ω_f2, ..ω_fN generated in said nonlinear optical medium from the second input port (20b) of said optical directional coupler, and wherein said carrier angular frequencies ω_Sj, ω_P1, ω_P2, and ω_fj are related by an expression: {K2(ωfj)+K2(ωSj)-K2(ωP1)-K2(ωP2)}L2 = {K1(ωfj)+K1(ωSj)-K1(ωP1)-K1(ωP2)}L1+ (2n±α)π where n is an integer and an allowable error α is in a range 0≤α<1/2.
An optical parametric circuit comprising:

a nonlinear optical medium (12) for generating optical waves having carrier angular frequencies ω_f1, ω_f2, ..ω_fN from optical waves having carrier angular frequencies ω_S1, ω_S2, ..ω_SN, ω_P1, and ω_P2, where ω_fj=ω_P1+ω_P2-ω_Sj(j=1,2,..N), characterised by

an optical directional coupler (20) having a first input port (20a), second input port (20b), first output port (20c) and second output port (20d), for dividing optical waves having carrier angular frequencies ω_Sj, ω_P1, ω_P2 and ω_fj into two optical waves of equal intensities and polarization states, respectively;

a first dispersive optical medium (21) having a length dimension L₁ and a propagation constant K₁(ω), for connecting the first output port (20c) of said optical directional coupler with one end of said nonlinear optical medium;

a second dispersive optical medium (22) having a length dimension L₂ and a propagation constant K₂(ω), for connecting the second output port (20d) of said optical directional coupler with other end of said nonlinear optical medium;

wherein pump waves having carrier angular frequencies ω_P1, ω_P2 are injected into the first input port (20a) of said optical directional coupler and signal waves having carrier angular frequencies ω_S1, ω_S2, ..ω_SN are injected into the second input port (20b) of said optical directional coupler so as to output pump waves having carrier angular frequencies ω_P1, ω_P2 together with four-wave-mixing waves having carrier angular frequencies ω_f1, ω_f2, ..ω_fN generated in said nonlinear optical medium from the first input port (20a) of said optical directional coupler and to output said signal waves having carrier angular frequencies ω_S1, ω_S2, ..ω_SN amplified in said nonlinear optical medium from the second input port (20b) of said optical directional coupler, and wherein said carrier angular frequencies ω_Sj, ω_P1, ω_P2, and ω_fj are related by an expression: {K2(ωfj)+K2(ωSj)-K2(ωP1)-K2(ωP2)}L2 = {K1(ωfj)+K1(ωSj)-K1(ωP1)-K1(ωP2)}L1+(2n-1±α)π where n is an integer and an allowable error α is in a range 0≤α<1/2.
An optical parametric circuit as claimed in one of claims 1 to 3, characterized in that the carrier angular frequencies ω_Sj(j=1, 2, ...N) of said signal waves and the carrier angular frequencies ω_P1 and ω_P2 of said pump waves are related by an expression: ωP1 < ωSj < ωP2 .
An optical parametric circuit as claimed in one of claims 1 to 3, characterized in that said optical directional coupler (23), said nonlinear optical medium (24) and said first and second dispersive optical media (25, 26) are polarization-maintaining type.
An optical parametric circuit as claimed in one of claims 1 to 3, characterized in that said circuit is provided with a compensating means (27) for compensating polarization dispersions in said optical directional coupler, said nonlinear optical medium and said first and second dispersive optical media.
An optical parametric circuit as claimed in claim 5, characterized in that two components of each electrical field of said signal waves, said pump waves and said four-wave-mixing waves polarized along two optic axes of said circuit are interchanged from one optic axis to another optic axis at points in said circuit which compensate total polarization dispersion in said circuit.
An optical parametric circuit as claimed in one of claims 1 and 2, characterized in that said pump waves are injected so as to equalize conversion gains (expressed as four-wave-mixing wave intensity/signal wave intensity) in two optic axes of said nonlinear optical medium.
An optical parametric circuit as claimed in one of claims 2 and 3, characterized in that said pump waves are injected so as to equalize signal wave amplification gains in two optic axes of said nonlinear optical medium.
An optical parametric circuit as claimed in one of claims 1 to 3, characterized in that said signal waves or said pump waves are angular-modulated.
An optical parametric circuit as claimed in claim 10, characterized in that amounts of angular modulation _Sj(t)(j=1, 2,..N) of said signal waves at time t and amounts of angular modulations _P1(t) and _P2(t) of said pump waves at time t are related by an expression: P1(t)+P2(t)-Sj(t)=0
An optical parametric circuit as claimed in one of claims 1 to 3, characterized in that optical intensities of said pump waves are modulated synchronously with optical intensities of said signal waves.
An optical parametric circuit as claimed in claim 1, characterized in that said circuit is provided with a multiplexing means (11) for multiplexing said signal waves and said pump waves; and
output waves from said multiplexing means are input into the first input port of said optical directional coupler.
An optical parametric circuit as claimed in claim 1, characterized in that said circuit is provided with a blocking means (28) for blocking said signal waves or said pump waves output from the first input port of said optical directional coupler.
An optical parametric circuit as claimed in one of claims 2 and 3, characterized in that said circuit is provided with a blocking means (28) for blocking said pump waves or said four-wave-mixing waves output from the first input port of said optical directional coupler.
An optical parametric circuit as claimed in one of claims 1 and 2, characterized in that said circuit is provided with a filtering means (13) for passing only said four-wave-mixing waves output from the second input port of said optical directional coupler.
An optical parametric circuit as claimed in one of claims 2 and 3, characterized in that said circuit is provided with a filtering means (29) for passing only said signal waves output from the second input port of said optical directional coupler.
An optical parametric circuit as claimed in claim 2, characterized in that said circuit is provided with a means (38) for separating said signal waves input into the second input port of said optical directional coupler, from said signal waves and said four-wave-mixing waves output from the second input port of said optical directional coupler.
An optical parametric circuit as claimed in claim 2, characterized in that said circuit is provided with a means (30) for separating said signal waves and said four-wave-mixing waves, both said signal waves and said four-wave-mixing waves being output from the second input port of said optical directional coupler.
An optical parametric circuit as claimed in claim 3, characterized in that said circuit is provided with a means (38) for separating said signal waves input into the second input port of said optical directional coupler from said signal waves output from the second input port of said optical directional coupler.
An optical circuit characterized by comprising:

a wavelength-division multiplexer (11) for multiplexing an optical input signal having a carrier angular frequency ω_S and optical gate signals having carrier angular frequencies ω_P1k, ω_P2k synchronized to said optical input signal and excited in a kth timeslot (k is an integer) of said optical input signal;

an optical parametric circuit claimed in claim 1 for inputting optical waves output from said wavelength-division multiplexer to the first input port of said optical directional coupler and outputting optical waves having a carrier angular frequency ω_fk(=ω_P1k+ ω_P2k- ω_S) in said kth timeslot from said second input port of said optical directional coupler; and

a wavelength-division demultiplexer (49) for separating said optical waves having carrier angular frequency ω_fk output from said optical parametric circuit for each k.
An optical circuit characterized by comprising a multi-stage serial circuit arrangement of the optical parametric circuit claimed in claim 13, wherein:

a 1-stage optical parametric circuit receives an optical input signal having a carrier angular frequency ω_S0 and first optical gate signals having carrier angular frequencies ω_{P1 1}, ω_{P2 1} synchronized to said optical input signal, and outputs an optical output signal having carrier angular frequency ω_{S 1} (=ω_{P1 1} + ω_P2 -ω_{S 0}); and

an n-stage optical parametric circuit (n is an integer of two or more) receives an optical output signal having a carrier angular frequency ω_{S n-1} output from a (n-1)-stage optical parametric circuit and nth optical gate signals having carrier angular frequencies ω_{P1 n}, ω_{P2 n} synchronized to said optical output signal having carrier angular frequency ω_{S n-1}, and outputs an optical output signal having carrier angular frequency ω_{S n}(=ω_{P1 n} +ω_{P2 n} -ω_{S n-1}).
An optical circuit characterized by comprising a multi-stage serial circuit arrangement of optical parametric circuit claimed in claim 19, wherein:

a 1-stage optical parametric circuit receives an optical input signal having a carrier angular frequency ω_{S 0} and first optical gate signals having carrier angular frequencies ω_{P1 1}, ω_{P2 1} synchronized to said optical input signal, and outputs an optical output signal having carrier angular frequency ω_{S 1} (=ω_{P1 1} + ω_{P2 1} -ω_{S 0}); and

an n-stage optical parametric circuit (n is an integer of two or more) receives an optical output signal having a carrier angular frequency ω_{S n-1} output from a (n-1)-stage optical parametric circuit and nth optical gate signals having carrier angular frequencies ω_{P1 n}, ω_{P2 n} synchronized to said optical output signal having carrier angular frequency ω_{S n-1} and outputs an optical output signal having carrier angular frequency ω_{S n}(=ω_{P1 n} + ω_{P2 n} -ω_{S 0}).
An optical circuit characterized by comprising a multi-stage serial circuit arrangement of optical parametric circuit claimed in claim 19, wherein:

a 1-stage optical parametric circuit receives an optical input signal having a carrier angular frequency ω_{S 0} and first optical gate signals having carrier angular frequencies ω_{P1 1}, ω_{P2 1} synchronized to said optical input signal, and outputs an optical output signal having carrier angular frequency ω_{S 0} and an optical output signal having carrier angular frequency ω_{S 1} (=ω_{P1 1} + ω_{P2 1}- ω_{S 0}); and

an n-stage optical parametric circuit (n is an integer of two or more) receives an optical output signal having a carrier angular frequency ω_{S 0} output from a (n-1)-stage optical parametric circuit and nth optical gate signals having carrier angular frequencies ω_{P1 n}, ω_{P2 n} synchronized to said optical output signal having carrier angular frequency ω_{S 0}, and outputs an optical output signal having carrier angular frequency ω_{S 0} and an optical output signal having carrier angular frequency ω_{S n}(=ω_{P1 n} ⁺ ω_{P2 n} -ω_{S 0}).
An optical circuit characterized by comprising a multi-stage serial circuit arrangement of optical parametric circuit claimed in claim 18, wherein:

a 1-stage optical parametric circuit receives an optical input signal having a carrier angular frequency ω_S and first optical gate signals having carrier angular frequencies ω_P+Δω₁, ω_P-Δω₁ synchronized to said optical input signal, and outputs optical output signals having carrier angular frequencies ω_S and ω_f(=2ω_P-ω_S); and

an n-stage optical parametric circuit (n is an integer of two or more) receives said optical output signals having carrier angular frequencies ω_S and ω_f output from a (n-1)-stage optical parametric circuit and nth optical gate signals having carrier angular frequencies ω_P+Δω_n, ω_P-Δω_n synchronized to said optical output signals output from said (n-1)-stage optical parametric circuit, and outputs optical output signals having carrier angular frequencies ω_S and ω_f, and

said optical circuit further comprising a means (13) for filtering only an optical wave having a carrier angular frequency ω_f from optical output signals of a final-stage optical parametric circuit.
An optical circuit characterized by comprising:

an optical parametric circuit claimed in one of claims 1 and 2 for receiving measured optical pulses having a carrier angular frequency ω_S and sampling optical pulses having a carrier angular frequency ω_P whose pulse width is not more than a pulse width of said measured optical pulses, and outputting an optical wave of a carrier angular frequency ω_f(=2ω_P-ω_S);

a photodetector (40) for detecting said optical wave having a carrier angular frequency ω_f; and

a display means (41) for displaying an output signal from said photodetector.